Increasing environmental restrictions and regulations are causing diesel engine manufacturers and packagers to develop technologies that improve and reduce the impact that operation of such engines have on the environment. As a result, much design work has gone into the controls that operate the combustion process within the engine itself in an attempt to increase fuel economy and reduce emissions such as NOx and particulate matter (e.g. soot). However, given the operating variables and parameters over which a diesel engine operates and given the tradeoff between NOx and particulate generation, many engine manufacturers and packagers have found it useful or necessary to apply exhaust after-treatment devices to their systems. These devices are used to filter the exhaust gas flow from the diesel engine to remove or reduce to acceptable levels certain emissions. Such devices are particularly useful in removing exhaust particulates, or soot, from the exhaust gas flow before such soot is released into the environment.
One such exhaust after-treatment device is called a Diesel Particulate Filter (DPF). The DPF is positioned in the exhaust system such that all exhaust gases from the diesel engine flow through it. The DPF is configured so that the particles in the exhaust gas are deposited in the filter substrate of the DPF. In this way, the particulates are filtered out of the exhaust gas so that the engine or engine system can meet or exceed the environmental regulations that apply thereto.
While such devices provide a significant environmental benefit, as with any filter, problems may occur as the DPF continues to accumulate these particulates. After a period of time the DPF becomes sufficiently loaded with soot that the exhaust gases experience a significant pressure drop passing through the increasingly restrictive filter. As a result of operating with an overly restrictive filter, the engine thermal efficiency declines because the engine must work harder and harder simply to pump the exhaust gases through the loaded DPF. Besides the reduced thermal efficiency, a second and potentially more dangerous problem may occur. Because the particulates such as soot accumulated in the DPF are flammable, continued operation with a loaded DPF raises the serious potential for uncontrolled exhaust fires if and when the accumulated soot is eventually ignited and burns uncontrollably.
To avoid either occurrence the engine packager typically incorporates one of several possible filter heating devices upstream of the DPF to periodically clean the filter. These filter heating devices are used periodically to artificially raise the temperature of the exhaust stream to a point at which the accumulated particulates will self-ignite. When initiated at a time before loading of the DPF becomes excessive, the ignition and burn off will occur in a safe and controlled fashion. This process of burning the particulate matter in such a controlled manner is called regeneration. The control of the method to generate the supplemental heat necessary to increase the temperature in the DPF is critical to the safe and reliable regeneration. Typically the acceptable regeneration range is from 600 to 900° C. Temperatures below this range are insufficient to ignite the accumulated particulate matter, and temperatures above this range may cause thermal damage to the filter media.
The rate at which particulates accumulates in the filter depends entirely upon the operating regime of the engine and the engine manufacturer or packager must also determine when to initiate the regeneration process. If regeneration is initiated too soon when the DPF is only lightly loaded, the process will be inefficient. If the regeneration is not initiated until the DPF is heavily loaded, the overall engine efficiency would have been unduly reduced as discussed above and there is a risk that the particulate matter may self-ignite and/or that the burn may be unsafe and uncontrolled.
In an attempt to properly determine when to initiate the regeneration process, several sensors and control algorithms have been developed. These sensors and control algorithms are used to estimate the particulate or soot loading of the DPF so that regeneration can be initiated only after particulate loading could cause an engine efficiency reduction but before excessive loading occurs actually resulting in such an efficiency reduction and raising the potential for self-ignition.
Besides determining when to initiate regeneration of a downstream DPF, monitoring particulate production within an exhaust stream can also provide engine operating feed back to the operator. Particulate matter is typically formed when an engine is running fuel rich, i.e. too much fuel is being injected into the engine such that it cannot be entirely combusted during the combustion cycle. Thus, a high level of particulate production sensed within the exhaust stream can indicate that the engine is running fuel rich and thus wasting fuel or at less than optimum condition.
Several downfalls exist with regard to current particulate sensing technology.
A first disadvantage of current sensors is the level of the signals generated by the sensors. Systems that measure the collection of particle charges on an electrode within the exhaust flow must measure very small amplitude electrical signals within the nano-ampere to micro-ampere range. This low signal level requires extreme amplification and conditioning which is not practical for application on vehicle systems.
A second disadvantage of current sensors relates to maintaining the electrode surface required for signal quality. Resistive heaters, catalyzed surfaces, insulated surfaces and spark cleaning have been used for cleaning of the electrode surfaces or attempting to prevent particulate build-up. These cleaning methods add cost and complexity to the design of the sensor.
A third disadvantage of current sensors relates to requirement of complex algorithms to make use of the electrical signal from the sensor. High speed analysis of spark discharges or small charge transfer often must incorporate compensation for exhaust and engine parameters such as temperature, humidity, fuel type, RPM, etc. This may result in a signal that is not robust and may be easily altered by conditions of the systems or may require exhaustive calibration. Further, this also adds to the increased processing power required to process the information.